Various forms of power steering systems are known. A typical hydraulic power steering system is disclosed in Japanese patent laid open (kokai) publication No. 60-229871. According to this known power steering system, a pinion connected to a steering wheel meshes with a rack, and a power cylinder is connected to the rack. The hydraulic pressure supplied to the power cylinder is controlled according to the reaction acting on the steering wheel.
This steering system includes, as an essential component, a rotary valve unit which can vary a cross sectional area of an oil passage defined between an input shaft and an annular valve element fitted on the input shaft according to the relative twisting angle between the input shaft and the valve element caused by the reaction from the road surface to the steering effort applied to the steering wheel. The rotary valve also supplies hydraulic pressure to either one of the two chambers of the power cylinder depending on the direction of the steering effort. Thus, the rotary valve supplies an appropriate hydraulic pressure to a selected one of the two chambers of the power cylinder according to the direction and the magnitude of the steering effort.
The steering wheel and the pinion are connected by a torsion bar pinion shaft to produce the relative twisting angle between them according to the applied steering effort. The twisting rigidity of this pinion shaft is known to strongly affect the handling of the vehicle, and an optimum value of the twisting rigidity depends on the vehicle speed, the steering angle and the steering angular speed. For instance, when the rigidity of the torsion bar is high, a larger steering effort is required for producing a given twisting angle, and the responsiveness of the rotary valve drops. This condition is undesirable when the vehicle speed is low and a greater assisting torque is required. Conversely, when the rigidity of the torsion bar is low, the responsiveness of the rotary valve increases, but this condition is not desirable when the vehicle is travelling at high speed, and a small steering angle causes a pronounced yaw movement of the vehicle.
To overcome this problem, the above mentioned conventional hydraulic steering system is provided with means for applying an opposing torque to the steering wheel shaft which increases with the increase in the travelling speed of the vehicle so that the vehicle operator may encounter a progressively greater steering reaction in turning the steering wheel as the vehicle speed increases. However, according to this solution to the problem, an artificial resistance to the steering effort is created, and the vehicle operator is prevented from getting information on the road condition and the handling of the vehicle which is transmitted to the steering wheel in the form of steering reaction.